1. Field of the Invention
The present invention relates to a driver circuit for driving an optical modulator or a semiconductor laser, and more specifically, to a driver circuit that can maintain favorable output wave form even when an electric wave form wherein a cross point is deviated from 50% is outputted.
2. Background Art
In optical communication systems, further higher-speed, higher-density, and longer-distance transmission of information have been demanded. For higher-speed transmission, the operating frequency of applications has been raised to 10 Gbps, 40 Gbps, and 100 Gbps. For higher-density transmission, multiplexed communications such as DWDM have been used. For relatively short-distance transmission, the semiconductor laser of direct modulator driving has been adopted; and for long-distance transmission, external modulator driving has been adopted.
In external modulator driving, electric absorption modulators (EAM) are mainly used. Change in optical output power of the EAM is non-linear to change in input voltage. Therefore, the driver circuit for driving the EAM must be capable of outputting electric wave form wherein the duty of output wave form is deviated, specifically, the cross point is deviated from 50% (for example, refer to Japanese Patent Application Laid-Open No. 11-14951). The adjustment of the cross point is also useful for improving the optical wave form of the semiconductor laser.